Bruckner was born in Hollywood, California to a Hungarian father and a Russian mother who have since divorced; her paternal grandfather was German. Her parents met in Hungary and immigrated to the U.S. in 1984 through a refugee camp in Italy. Agnes Bruckner has two sisters and a brother.
Bruckner speaks some Russian and is fluent in Hungarian, having grown up speaking the language. Agnes Bruckner has been involved in dance, ballet, and tap since the age of five and initially wanted to pursue a career as a dancer. At the age of eight, Bruckner worked as a child model at the suggestion of her mother, and also appeared in a beauty pageant. Bruckner grew up in Los Feliz, Los Angeles, California and then lived in Portland, Oregon from age five to 10. Agnes Bruckner returned with her family to Los Angeles to pursue an acting career, moving to Burbank, California.